[mms-lib] Additional check for Message-ID in M-Retrieve.conf

Message-ID must be present only if the M-Retrieve.conf PDU
includes the requested MM. If it doesn't include the message
then X-Mms-Retrieve-Status should indicate the error but just
in case let's check Message-ID as well.
Merge pull request #46 from monich/msgid

Additional check for Message-ID in M-Retrieve.conf